Method and apparatus for packet transmission in carrier-sense-multiple-access network

ABSTRACT

A communication method for a CSMA network including a radio terminal and a base station supporting RTS/CTS, includes: the base station transmitting an RTS frame to the radio terminal during a transmission-suspend-period in which the radio terminal suspends transmission to prevent a collision of packets; the radio terminal transmitting an RTR frame to the base station after the transmission-suspend-period has elapsed; and the base station transmitting a data frame to the radio terminal in response to the RTR frame.

TECHNICAL FIELD

The present invention relates to a communication method for a digitalradio communication system adopting carrier-sense multiple access(CSMA), which is one of the random access technologies, and morespecifically, the invention relates to a communication method capable ofavoiding a collision of wireless packets due to the influence of ahidden terminal.

BACKGROUND ART

The conventional communication method will be explained first. The CSMA,which is one of the random access technologies, is adopted, for example,in a communication system performing wireless packet communication andthe like. In the CSMA, a plurality of radio terminals constituting thesystem performs carrier-senses for a radio channel prior to wirelesspacket transmission. When it is recognized that the channel is beingused (channel busy), transmission of the wireless packet is suspended,and thereafter, when it is recognized that the channel is not being used(channel idle), the wireless packet is transmitted.

In this communication system, however, there are cases that atransmission signal from another radio terminal cannot be directlyreceived, such as a case that the radio terminals are located with sucha distance therebetween that radio waves do not reach, or a case thatthere is an obstacle, which blocks radio waves, between the radioterminals. The radio terminal whose presence cannot be recognized, inspite of constituting the same communication system, is referred to as a“hidden terminal”. Since the carrier sense does not work effectivelybetween the radio terminals corresponding to the hidden terminals, thereis a case that one terminal starts transmission of a wireless packet,while the other is transmitting a wireless packet, and in such a case,for example, a collision of wireless packets occurs in a radio basestation located at an intermediate position between the radio terminals,thereby disabling normal communication.

In Wireless LAN Standard IEEE802.11, the problem of hidden terminals isremedied by request-to-send (RTS)/clear-to-send (CTS) of a distributedcoordination function (DCF) using the carrier sense multipleaccess/collision avoidance (CSMA/CA).

A communication method between a base station (AP) and a radio terminal(STA) in a wireless LAN system adopting the CSMA will be explainedbelow. It is assumed here that communication is performed in such astate that STA (1) belongs to AP (1), and STA (2) belongs to AP (2). TheSTA (2) is in a communication range of the STA (1), and the STA (1) isin an interference range of the STA (2). A packet format of the RTS andthe CTS used in the system includes a packet type field fordiscriminating the packets of RTS and CTS from each other, a destinationaddress field, a sender address field (not in the CTS), a channel useperiod field by a transmitted wireless packet, and anerror-checking-code field for checking a bit error in the packet.

Firstly, the AP (1) transmits an RTS frame, which is a control frame, tothe STA (1). The STA (1) transmits a CTS frame to the AP (1). Therespective frames include virtual carrier sense information referred toas a net allocation vector (NAV), indicating, for example, the channeluse period in the communication with a radio terminal corresponding tothe destination address. Therefore, transmission from a radio terminalother than the destination address is suspended until the time (period)specified in the NAV. That is, here, the STA (2) is in thetransmission-suspended state.

The AP (1) having received the CTS frame then transmits a DATA frame tothe STA (1). The STA (1) having received the DATA frame sends back anACK frame to the AP (1).

On the other hand, the STA (2) having received the CTS frame from theSTA (1) cannot send back the CTS frame even when it has received the RTSframe from the AP (2), since it is in the transmission-suspended stateby the NAV. Since the CTS frame is not sent back from the STA (2), theAP (2) retransmits the RTS frame until the transmission-suspended stateof the STA (2) is cancelled. When the number of retransmission reaches apreset upper limit, the AP (2) can cancel the frame.

When the AP (2) retransmits the RTS frame to the STA (2) in the statethat the channel use period is expired and the transmission-suspendedstate is cancelled, the STA (2) transmits the CTS frame to the AP (2).The AP (2) having received the CTS frame transmits a DATA frame to theSTA (2), and the STA (2) having received the DATA frame sends back theACK frame to the AP (2).

Thus, in the conventional wireless LAN system, a plurality of radioterminals is connected to a plurality of base stations operating in thesame frequency, and when there is a hidden terminal due to theinterference between the radio terminals connected to different basestations, a collision of packets can be avoided by the RTS/CTS.

In the conventional communication method, however, for example, the STA(2) is turned to the transmission-suspended state due to the NAV of theSTA (1) connected to the other AP (1) operating in the same frequency.Therefore, in the transmission-suspended state, the STA (2) cannottransmit the ACK frame, even when it can receive the DATA frame from theAP (1), thereby causing a drop in the throughput considerably.

As a method of avoiding a drop in the throughput, for example, there area method of controlling a downlink and an uplink timewise bysynchronizing a plurality of base stations, and a method of adjustingthe distance between the base stations. However, with these methods,there is a problem in that a plurality of companies or individualscannot install the base station at random.

The present invention has been achieved in order to solve the aboveproblems. It is an object of the present invention to provide acommunication method capable of avoiding a drop in the throughput,without performing synchronous control between the base stations anddistance adjustment between the base stations.

DISCLOSURE OF INVENTION

A communication method according to the present invention, which is acommunication method for a carrier-sense-multiple-access (CSMA) networkincluding a radio terminal and a base station to which the radioterminal belongs and supports request-to-send/clear-to-send (RTS/CTS) toprevent a collision of packets due to a hidden terminal, includes:RTS-transmitting including the base station transmitting arequest-to-send (RTS) frame to the radio terminal during atransmission-suspend-period in which the radio terminal suspendstransmission to prevent the collision of packets; RTR-transmittingincluding the radio terminal transmitting a request-to-receive (RTR)frame to the base station after the transmission-suspend-period haselapsed; and data-transmitting including the base station transmitting adata frame to the radio terminal in response to the RTR frame.

According to the present invention, when a particular radio terminal isin the transmission-suspended state due to the influence of a hiddenterminal, although there is an access from the base station, the radioterminal transmits the RTR frame for requesting the base station toretransmit the DATA frame, which could not be received before, at thetime of shifting to the transmission-enabled state, and the base stationretransmits the past DATA frame.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts the configuration of a communication system capable ofrealizing a communication method according to the present invention;

FIG. 2 depicts a format of a request-to-receive (RTR) frame;

FIG. 3 depicts a frame format of a request-to-send (RTS) frame;

FIG. 4 depicts a frame format of a clear-to-send (CTS) frame;

FIG. 5 depicts a communication method according to a first embodiment;

FIG. 6 depicts a communication method according to a second embodiment;

FIG. 7 depicts the configuration of a communication system capable ofrealizing the communication method according to the present invention;

FIG. 8 depicts a communication method according to a third embodiment;

FIG. 9 depicts a communication method according to a fourth embodiment;

FIG. 10 depicts the configuration of a communication system capable ofrealizing the communication method according to the present invention;

FIG. 11 depicts a communication method according to a fifth embodiment;

FIG. 12 depicts the configuration of the communication system capable ofrealizing a communication method according to the present invention; and

FIG. 13 depicts a communication method according to a sixth embodiment.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Exemplary embodiments of a communication method according to the presentinvention will be explained below in detail with reference to theaccompanying drawings.

FIG. 1 depicts the configuration of a communication system capable ofrealizing the communication method according to the present invention,in which the communication status between a base station (AP) and aradio terminal (STA) is shown. In this communication system, a case thatpacket communication is carried out in the state that an STA3 belongs toan AP1, and an STA4 and an STA5 belong to an AP2 is assumed. The STA4 islocated in the communication range of the STA3, and the STA3 is locatedin an interference range of the STA4.

FIG. 2 depicts a request-to-receive (RTR) format according to a firstembodiment of the present invention. The RTR frame includes a packettype field for discriminating the packets (RTR, RTS, and CTS) from eachother, a destination address field, a sender address field, a channeluse period field indicating the use period of a channel by the wirelesspacket, and an error-checking-code field added with a calculation resultfor checking a bit error in the packet. FIG. 3 depicts a frame format ofthe RTS as in the conventional method, and FIG. 4 depicts a frame formatof the CTS as in the conventional method. The RTS frame and the CTSframe respectively include a packet type field for discriminating thepackets, a destination address field, a sender address field (not in theCTS), a channel use period field indicating the use period of a channelby the wireless packet, and an error-checking-code field added with acalculation result for checking a bit error in the packet. Detailedfields in IEEE802.11 and fields commonly added to the respectivewireless packets depending on modulation and demodulation methods andthe like are omitted.

The communication method according to the first embodiment will bespecifically explained below with reference to the accompanyingdrawings. FIG. 5 depicts the communication method according to the firstembodiment. An example of a communication method between a base station(AP) and a radio terminal (STA) in a wireless LAN system in conformitywith IEEE802.11 will be explained below. A time axis is plotted on theY-axis, and arrows express the shift of a frame from the AP to the STA,or a frame from the STA to the AP.

Firstly, the AP1 transmits an RTS frame, which is a control frame inIEEE802.11, to the STA3 (step S1 in FIG. 5). The STA3 transmits a CTSframe to the AP1 (step S2). The respective frames include virtualcarrier sense information referred to as NAV, which indicates a channeluse period in the communication with, for example, a radio terminalcorresponding to the destination address. Therefore, radio terminalsother than the destination address are turned to atransmission-suspended state until the time specified in the NAV. Thatis, since the STA4 is in the communication range of the STA3, the STA4is turned to the transmission-suspended state (step S3).

The AP1 having received the CTS frame transmits a DATA frame to the STA3(step S4). The STA3 having received the DATA frame sends back an ACKframe to the AP1 (step S5). According to the IEEE802.11, RTS/CTS is usedas the method of solving the problem of a hidden terminal.

On the other hand, the STA4 having received the CTS frame from the STA3in the processing at step S2 is turned to the transmission-suspendedstate (step S3). Therefore, even when having received the RTS frame fromthe AP2 (step S6), the STA4 cannot send back the CTS frame to the AP2(step S7). Since the CTS frame is not sent back to the AP2 even afterthe predetermined time has passed, the AP2 retransmits the RTS frame(step S8). The STA4 cannot send back the CTS frame to the AP2, since itis still in the transmission-suspended state (step S9).

In the first embodiment, when the number of retransmission reaches two,the AP2 temporarily stores the DATA frame for the STA4, for example, ina predetermined buffer (step S10). If there is DATA for another STA, theAP2 handles the communication with the other STA preferentially. Asshown in FIG. 5, the AP2 gives priority to the communication with anSTA5, and transmits the RTS frame to the STA5 (step S11). On the otherhand, for example, when the channel use period is ahead of the timeindicated by the CTS frame at step S2, according to the NAV included inthe RTS frame at step S11 (step S12), the STA4 extends thetransmission-suspend-period. While a case that the number ofretransmission is two is explained in the first embodiment as anexample, the number of retransmission is not limited thereto.

The STA5 having received the RTS frame in the processing at step S11transmits the CTS frame to the AP2 (step S13). The AP2 having receivedthe CTS frame transmits the DATA frame to the STA5 (step S14). On theother hand, STA4 extends the transmission-suspend-period based on theNAV included in the DATA frame addressed to the STA5 (step S15).Thereafter, the STA5 having received the DATA frame sends back the ACKframe to the AP2 (step S16). At this point in time, thetransmission-suspended state of the STA4 renewed at step S15 iscancelled, and the STA4 is turned to a transmission-enabled state.

The STA4 could not transmit the CTS frame due to the influence of ahidden terminal in spite of having received the RTS frame from the AP2in the past (steps S6 and S8). Therefore, the STA4 now in thetransmission-enabled state transmits the RTR frame for requestingretransmission of the RTS frame to the AP2 (step S17). On the otherhand, the STA3 is turned to the transmission-suspended state, forexample, until the reception finish time of the CTS frame from the STA4,based on the NAV included in the RTR frame at step S17 (step S18).

The AP2 having received the RTR frame in the processing at step S17transmits the RTS frame to the STA4 in response thereto (step S19). Onthe other hand, the STA5 is turned to the transmission-suspended state,for example, until the reception processing of the STA4 finishes, basedon the NAV included in the RTS frame at step S19 (step S20).

The STA4 having received the RTS frame in the processing at step S19transmits the CTS frame to the AP2 (step S21). On the other hand, STA3extends the transmission-suspend-period based on the NAV in the CTSframe to the AP2 (step S22).

The AP2 having received the CTS frame in the processing at step S21reads the DATA frame temporarily stored in the processing at step S10and transmits the DATA frame to the STA4 (step S23). Lastly, the STA4having received the DATA frame sends back the ACK frame to the AP2 (stepS24). At this point in time, the transmission-suspended state of theSTA3 and the STA5 is cancelled, and changed to the transmission-enabledstate.

In the first embodiment, a case that a hidden terminal appears due to aradio terminal connected to the base station operating in the samefrequency has been explained. However, the present invention is notlimited thereto, and for example, when a radio terminal cannot performtransmission processing since the base station operating in the samefrequency operates in the interference range, the similar procedure canbe applied. Furthermore, even when a radio terminal receives a framefrom the base station connected by carrier sense but is turned to thetransmission-suspended state immediately after receiving the frame, thesame procedure can be applied by transmitting the RTR frame when theradio terminal is turned to the transmission-enabled state.

Even in a sequence in which the RTS/CTS procedure is not carried out asthe measure against the hidden terminal, the similar procedure can beapplied by transmitting the RTR frame. In this case, the processing atsteps S6 to S9 is replaced by the retransmission processing of the DATAframe, and after the transmission of the RTR frame, only the processingat step S23 (DATA frame) and at step S24 (ACK frame) is carried out.Furthermore, the similar procedure can be applied not only to the upwardtraffic from the radio terminal, but also to the downward traffic fromthe base station. In the configuration of the communication system, itis assumed herein that the base station can be a particular radioterminal, and a radio terminal can be the base station. While the accessmethod based on the CSMA/CA has been explained in this embodiment, thesimilar procedure can be also applied to an access method based onpolling control, by reserving polling time within the RTR frame andreserving delivery of the reception frame.

Thus, in the first embodiment, when a particular radio terminal is inthe transmission-suspended state due to the influence of a hiddenterminal, in spite of having an access from the base station, the radioterminal transmits the RTR frame for requesting the base station toretransmit the DATA frame, which the radio terminal could not receive,when the radio terminal is turned to the transmission-enabled state, sothat the base station retransmits the past DATA frame. Accordingly,since the downlink packet from the base station can be efficientlyreceived, a considerable drop in the throughput can be avoided withoutestablishing synchronization of downlink/uplink between the basestations. Furthermore, since this method has compatibility with theIEEE802.11, a conventional WLAN card can be used.

A communication method according to a second embodiment of the presentinvention will now be explained. Since the configuration of thecommunication system is the same as that of FIG. 1 in the firstembodiment, like reference numerals are designated with like parts, andexplanation thereof is omitted. The respective frame formats used in thesecond embodiment are the same as those of FIGS. 2, 3, and 4 in thefirst embodiment.

The communication method according to the second embodiment will beexplained specifically, with reference to the accompanying drawings.FIG. 6 depicts the communication method according to the secondembodiment only the operation different from that of the firstembodiment will be explained below.

Since the STA4 could not transmit the CTS frame due to the influence ofthe hidden terminal, in spite of having received the RTS frame from theAP2 in the past (steps S6 and S8), the STA4 now in thetransmission-enabled state in the processing at step S16 transmits theRTR frame to the AP2 for requesting the AP2 to retransmit the RTS frame(step S17). On the other hand, the STA3 is turned to thetransmission-suspended state, for example, until the receptionprocessing of the STA4 finishes, based on the NAV included in the RTRframe at step S17 (step S31).

The AP2 having received the RTR frame in the processing at step S17reads the DATA frame temporarily stored in the processing at step S10and transmits the DATA frame to the STA4 (step S23). On the other hand,the STA5 is turned to the transmission-suspended state, for example,until the reception processing of the STA4 finishes, based on the NAVincluded in the DATA frame at step S23 (step S20).

Lastly, the STA4 having received the DATA frame sends back the ACK frameto the AP2 (step S24). At this point in time, the transmission-suspendedstate of the STA3 and the STA5 is cancelled, and changed to thetransmission-enabled state.

Thus, in the second embodiment, when the base station receives the RTRframe, the RTS/CTS procedure executed in the first embodiment isomitted. Accordingly, the band is not occupied by the RTS/CTS procedure,thereby further preventing a drop in the throughput.

A communication method according to a third embodiment of the presentinvention will be explained next. FIG. 7 depicts the configuration of acommunication system capable of realizing the communication methodaccording to the present invention, wherein the communication statusbetween the AP and the STA, and the communication status of the AP usingthe same frequency are shown. In this communication system, a case thatpacket communication is carried out in the state that the STA3 belongsto the AP1, and the STA4 and the STA5 belong to the AP2 is assumed. TheAP2 is located in the communication range of the STA3, and the STA3 islocated in the interference range of the AP2. The respective frameformats used in the third embodiment are the same as shown in FIGS. 2,3, and 4 in the first embodiment.

The communication method according to the third embodiment will beexplained specifically with reference to the accompanying drawings. FIG.8 depicts the communication method according to the third embodiment.The communication method between the AP and the STA in the wireless LANsystem based on the IEEE802.11 will be explained below as an example.The time axis is plotted on the Y-axis, and arrows express the shift ofa frame from the AP to the STA, or a frame from the STA to the AP.

Firstly, the AP1 transmits the RTS frame, which is a control frame inIEEE802.11, to the STA3 (step S41 in FIG. 8). The STA3 transmits the CTSframe to the AP1 (step S42). Since the respective frames include thevirtual carrier sense information referred to as NAV, radio terminalsother than the destination address are turned to thetransmission-suspended state until the time specified in the NAV. Thatis, since the AP2 is in the communication range of the STA3, the AP2 isturned to the transmission-suspended state (step S43).

The AP1 having received the CTS frame transmits the DATA frame to theSTA3 (step S44). The STA3 having received the DATA frame sends back theACK frame to the AP1 (step S45). According to the IEEE802.11, RTS/CTS isused as the method of solving the problem of a hidden terminal.

On the other hand, the AP2 having received the CTS frame from the STA3in the processing at step S42 is turned to the transmission-suspendedstate (step S43). Therefore, even when having received the RTS framefrom the STA4 (step S46), the AP2 cannot send back the CTS frame to theSTA4 (step S7). Since the CTS frame is not sent back to the STA4 evenafter the predetermined time has passed, the STA4 retransmits the RTSframe (step S48). The AP2 cannot send back the CTS frame to the STA4 asin the last occasion, since it is still in the transmission-suspendedstate (step S49).

In the third embodiment, when the number of retransmission reaches two,the STA4 temporarily stores the DATA frame for the AP2, for example, ina predetermined buffer (step S50). While a case that the number ofretransmission is two is explained in the third embodiment as anexample, the number of retransmission is not limited thereto.Furthermore, if there is DATA for another radio terminal or basestation, the communication with the other radio terminal or base stationcan be carried out preferentially.

After the transmission-suspended state at step S43 is cancelled andchanged to the transmission-enabled state, the AP2, which could nottransmit the CTS frame in spite of having received the RTS frame fromthe STA4 in the past (steps S46 and S48), transmits the RTR frame to theSTA4 to request the STA4 to retransmit the RTS frame (step S51). On theother hand, the STA3 is turned to the transmission-suspended state basedon the NAV included in the RTR frame at step S51, for example, until thereception finish time of the CTS frame from the AP2, (step S52).

The STA4 having received the RTR frame in the processing at step S51transmits the RTS frame to the AP2 in response thereto (step S53). Onthe other hand, the STA5 is turned to the transmission-suspended state,for example, until the reception processing of the STA4 finishes, basedon the NAV included in the RTS frame at step S53 (step S54).

Subsequently, the AP2 having received the RTS frame in the processing atstep S53 transmits the CTS frame to the STA4 (step S55). On the otherhand, the STA3 extends the transmission-suspend-period, according toneed, based on the NAV in the CTS frame addressed to the STA4 (stepS56).

The STA4 having received the CTS frame in the processing at step S55reads the DATA frame temporarily stored in the processing at step S50and transmits the DATA frame to the AP2 (step S57). Lastly, the AP2having received the DATA frame sends back the ACK frame to the STA4(step S58). At this point in time, the transmission-suspended state ofthe STA3 and the STA5 is cancelled, and changed to thetransmission-enabled state.

In this embodiment, a case that a hidden terminal appears due to a radioterminal adjacent to the base station, which operates in the samefrequency, has been explained. However, for example, the same procedurecan be applied to a case that a radio terminal cannot performtransmission processing, because the base station operating in the samefrequency operates within the interference range. Furthermore, even whenthe base station receives a frame from a radio terminal connected bycarrier sense, but is turned to the transmission-suspended stateimmediately after receiving the frame, the same procedure can be appliedby transmitting the RTR frame when the base station is turned to thetransmission-enabled state.

Furthermore, even in a sequence in which the RTS/CTS procedure as themeasures against hidden terminals and the interference problem is notperformed, the same procedure can be applied by transmitting the RTRframe. In this case, the processing at steps S46 to S49 is replaced bythe retransmission processing of the DATA frame, and after thetransmission of the RTR frame, only the procedure at step S57 (DATAframe) and step S58 (ACK frame) is performed. Furthermore, while theaccess method based on the CSMA/CA has been explained in thisembodiment, the same procedure can be also applied to the access methodbased on the polling control, by reserving polling time within the RTRframe and reserving delivery of the reception frame. In this case, afield in which the delivery reservation is performed is added to the RTRframe. In the configuration of the communication system, it is assumedherein that the base station can be a particular radio terminal, and aradio terminal can be the base station, in the first and the secondembodiments.

In the third embodiment, when a particular base station is in thetransmission-suspended state due to the influence of a hidden terminalor interference, in spite of having an access from a radio terminal, thebase station transmits the RTR frame for requesting the radio terminalto retransmit the DATA frame, which could not been received, when thebase station is turned to the transmission-enabled state, so that theradio terminal retransmits the past DATA frame. Accordingly, an uplinkpacket from the radio terminal can be efficiently received.

A communication method according to a fourth embodiment of the presentinvention will now be explained. Since the configuration of thecommunication system is the same as that of FIG. 7 in the thirdembodiment, like reference numerals are designated with like parts, andexplanation thereof is omitted. The respective frame formats used in thefourth embodiment are the same as those of FIGS. 2, 3, and 4 in thefirst embodiment.

The communication method according to the fourth embodiment will beexplained specifically, with reference to the accompanying drawings.FIG. 9 depicts the communication method according to the fourthembodiment. Only the operation different from that of the thirdembodiment will be explained below.

After the transmission-suspended state at step S43 has been cancelled,the AP2 in the transmission-enabled state, which could not transmit theCTS frame in spite of having received the RTS frame from the STA4 in thepast (steps S46 and S48), transmits the RTR frame to the STA4 to requestthe STA4 to retransmit the RTS frame (step S51). On the other hand, theSTA3 is turned to the transmission-suspended state based on the NAVincluded in the RTR frame at step S51, for example, until the receptionprocessing of the AP2 finishes (step S61).

The STA4 having received the RTR frame in the processing at step S51reads the DATA frame temporarily stored at step S50, and transmits theDATA frame to the AP2 (step S57). On the other hand, the STA5 is turnedto the transmission-suspended state, for example, until the receptionprocessing of the AP2 finishes, based on the NAV included in the DATAframe at step S57 (step S62).

Lastly, the AP2 having received the DATA frame sends back the ACK frameto the STA4 (step S58), and at this point in time, thetransmission-suspended state of the STA3 and the STA5 is cancelled, andchanged to the transmission-enabled state.

Thus, in the fourth embodiment, when the radio terminal receives the RTRframe, the RTS/CTS procedure executed in the third embodiment isomitted. Accordingly, the band is not occupied by the RTS/CTS procedure,thereby further preventing a drop in the throughput.

A communication method according to a fifth embodiment of the presentinvention will be explained next. FIG. 10 depicts the configuration of acommunication system capable of realizing the communication methodaccording to the present invention, wherein the communication statusbetween the APs, and STAs belonging to the respective APs are shown. Inthe communication method between the AP and the STA is the same as inthe first to the fourth embodiments. In this embodiment, the relationbetween the AP and the STA will be explained, wherein the AP can be aparticular STA, or the STA can be an AP. The APs can be connected byanother network, or can be unconnected.

In this communication system, it is assumed that the base stationcommunicates with other base stations by using the same channel or achannel affected by interference. Specifically, an AP1 is located in theinterference range of an AP2, the AP2 is located in the interferenceranges of an AP6 and the AP1, and the AP6 is located in the interferencerange of the AP2. The respective frame formats used in the fifthembodiment are the same as shown in FIGS. 2, 3, and 4 in the firstembodiment.

The communication method according to the fifth embodiment will beexplained specifically with reference to the accompanying drawings. FIG.11 depicts the communication method according to the fifth embodiment.The communication method between the APs in the wireless LAN systembased on the IEEE802.11 will be explained below as an example.

Firstly, the STA3 transmits the RTS frame, which is a control frame inIEEE802.11, to the AP1 (step S71 in FIG. 11). The AP1 then transmits theCTS frame to the STA3 (step S72). Since the AP2 is within thecommunication range of the AP1, the AP2 is turned to thetransmission-suspended state according to the NAV (step S73).

The STA3 having received the CTS frame transmits the DATA frame to theAP1 (step S74). The AP1 having received the DATA frame sends back theACK frame to the STA3 (step S75).

On the other hand, the AP2 having received the CTS frame from the AP1 inthe processing at step S72 is turned to the transmission-suspended stateaccording to the NAV (step S73). Therefore, as shown in FIG. 11, evenwhen having received the frame for communication between base stationsfrom the AP6 (step S76), the AP2 cannot send back the response frame tothe AP6 (step S77). Since the response frame is not sent back to the AP6even after the predetermined time has passed, the AP6 retransmits theframe for communication between base stations (step S78). Since the AP2is still in the transmission-suspended state, the AP2 cannot send backthe response frame to the AP6 as in the last occasion (step S79).

In the fifth embodiment, when the number of retransmission reaches two,the AP6 temporarily stores the DATA frame for the AP2, for example, in apredetermined buffer (step S80). In the fifth embodiment, if there isDATA for another STA or AP, the communication with the other STA or APis carried out preferentially. As shown in the FIG. 11, thecommunication with the STA5 is given priority, and the AP2 transmits theRTS frame to the STA5 (step S81). On the other hand, when the channeluse period is ahead of the time indicated by the CTS frame at step S73based on the NAV included in the RTS frame at step S81, the AP2 extendsthe transmission-suspend-period (step S82). While a case that the numberof retransmission is two is explained in this embodiment as an example,the number of retransmission is not limited thereto.

Subsequently, the STA5 having received the RTS frame in the processingat step S81 transmits the CTS frame to the AP6 (step S83). The AP6having received the CTS frame transmits the DATA frame to the STA5 (stepS84). On the other hand, the AP2 extends thetransmission-suspend-period, according to need, based on the NAV in theDATA frame addressed to the STA5 (step S85). Thereafter, the STA5 havingcompleted reception of the DATA frame sends back the ACK frame to theAP6 (step S86). At this point in time, the transmission-suspended stateof the AP2 renewed at step S85 is cancelled, and the AP2 is turned totransmission-enabled state.

The AP2 in the transmission-enabled state, which could not transmit theresponse frame in spite of having received the frame for communicationbetween base stations from the AP6 in the past (steps S76 and S78),transmits the RTR frame to the AP6 to request the AP6 to retransmit theframe for communication between base stations (step S87). On the otherhand, the AP1 is turned to the transmission-suspended state based on theNAV included in the RTR frame at step S87, for example, until thereception finish time of the response frame from the AP2 (step S88).While frame transmission for the communication between base stations isimmediately requested by the RTR frame in this embodiment, a method ofreserving the reception time can be also used in the communicationsystem based on the polling method.

The AP6 having received the RTR frame in the processing at step S87transmits the frame for communication between base stations to the AP2(step S89). On the other hand, the STA5 is turned to thetransmission-suspended state, for example, until the receptionprocessing of the AP2 finishes, based on the NAV included in the framefor communication between base stations at step S89 (step S90). The AP2having completed reception of the frame for communication between basestations sends back the response frame to the AP6 (step S91), and atthis point in time, the transmission-suspended state of the STA5 and theAP1 is cancelled, and the STA5 and the AP1 are turned to thetransmission-enabled state.

Thus, in the fifth embodiment, when a particular radio terminal (or basestation) is in the transmission-suspended state, in spite of having anaccess from a base station (or a radio terminal), the radio terminal (orthe base station) transmits the RTR frame for requesting the basestation (or the radio terminal) to retransmit the frame (including DATAand frame for communication between base stations), which could not bereceived, when the radio terminal (or the base station) is turned to thetransmission-enabled state, so that the base station (or the radioterminal) retransmits the past frame. Accordingly, a packet can beefficiently received, thereby avoiding a considerable drop in thethroughput.

Even in a sequence in which the RTS/CTS procedure is not carried out asthe measure against the hidden terminal and the interference problem,the similar procedure can be applied by transmitting the RTR frame.While the access method based on the CSMA/CA has been explained in thefifth embodiment, the similar procedure can be also applied to theaccess method based on polling control, by reserving polling time withinthe RTR frame and reserving delivery of the reception frame. In thiscase, a field in which the delivery reservation is performed is added tothe RTR frame. While in the fifth embodiment, the RTR frame and the DATAframe between the base stations are used in the same frequency as thatof other frames, the RTR frame and the DATA frame can be also used inother frequencies.

A communication method according to a sixth embodiment of the presentinvention will be explained. FIG. 12 depicts the configuration of thecommunication system capable of realizing the communication methodaccording to the present invention, wherein the communication statebetween the AP and the STA is shown. The STA3 belongs to the AP1, andthe STA4 and the STA5 belong to the AP2, and communicate with eachother. In FIG. 12, the AP1 is located in the communication range of theSTA3, and the AP2 is located in the interference range of the STA3. Itis assumed herein that all terminals use the same channel.

The communication method according to the sixth embodiment will beexplained specifically with reference to the accompanying drawings. FIG.13 depicts the communication method according to the sixth embodiment.The communication method between the AP and the STA based on theIEEE802.11e, which is the standard for applying QoS to the wireless LAN,will be explained below as an example.

Firstly, the AP1 transmits the RTS frame, which is a control frame inthe IEEE802.11, to the STA3 (step S101 in FIG. 13). The STA3 thentransmits the CTS frame to the AP1 (step S102). Since the AP2 is withinthe communication range of the STA3, the AP2 is turned to thetransmission-suspended state according to the NAV (step S103).

The AP1 having received the CTS frame transmits the DATA frame to theSTA3 (step S104). The STA3 having received the DATA frame sends back theACK frame to the AP1 (step S105).

On the other hand, the AP2 having received the CTS frame from STA3 inthe processing at step S102 is turned to the transmission-suspendedstate according to the NAV (step S103). Therefore, as shown in FIG. 13for example, even when having received the RTS frame from the STA4 (stepS106), the AP2 cannot send back the CTS frame to the STA4 (step S107).Since the CTS frame is not sent back to the STA4 even after thepredetermined time has passed, the STA4 retransmits the RTS frame (stepS108). Since the AP2 is still in the transmission-suspended state, theAP2 cannot send back the CTS frame to the STA4 as in the last occasion(step S109).

In the sixth embodiment, when the number of retransmission reaches two,the STA4 temporarily stores the DATA frame for the AP2, for example, ina predetermined buffer (step S110). While an example in which the numberof retransmission is two is explained in the sixth embodiment, thenumber of retransmission is not limited to two. Furthermore, if there isDATA for another radio terminal or the base station, the communicationwith the other radio terminal or base station can be carried outpreferentially.

Since the AP2 is in the transmission-suspended state (step S103), evenwhen having received the RTS frame from the STA5 (step S111), the AP2cannot send back the CTS frame to the STA5 (step S112). Since the CTSframe is not sent back to the STA5 even after the predetermined time haspassed, the STA4 retransmits the RTS frame (step S113). The AP2 cannotsend back the CTS frame to the STA5 as in the last occasion, since it isstill in the transmission-suspended state (step S114).

When the number of retransmission reaches two, the STA5 temporarilystores the DATA frame for the AP2, for example, in a predeterminedbuffer (step S115).

After the transmission-suspended state at step S103 has been cancelled,the AP2 in the transmission-enabled state, which could not transmit theCTS frame in spite of having received the RTS frames from the STA4 andthe STA5 in the past (steps S106, S108, S111, and S113), transmits theRTR frame first to the STA5 to request the STA5 to retransmit the RTSframe (step S116). On the other hand, the STA3 is turned to thetransmission-suspended state based on the NAV included in the RTR frameat step S116, for example, until the reception finish time of the CTSframe from the AP2, (step S117). In the sixth embodiment, for theconvenience sake, the DATA frame to be transmitted from the STA5 to theAP2 is given priority. However, it is not limited thereto, and if theDATA frame to be transmitted from the STA4 to the AP2 has a higherpriority, the AP2 transmits the RTR to the STA4 first at step S116.

The STA5 having received the RTR frame in the processing at step S116transmits the RTS frame to the AP2 in response thereto (step S118). Onthe other hand, the STA4 is turned to the transmission-suspended state,for example, until the reception processing of the STA5 finishes, basedon the NAV included in the RTS frame at step S118 (step S119).

The AP2 having received the RTS frame in the processing at step S118transmits the CTS frame to the STA5 (step S120). On the other hand, theSTA3 extends the transmission-suspend-period, according to need, basedon the NAV in the CTS frame addressed to the STA5 (step S121).

The STA5 having received the CTS frame in processing at step S120 readsthe DATA frame temporarily stored in the processing at step S115 andtransmits the DATA frame to the AP2 (step S112). Lastly, the AP2 havingreceived the DATA frame sends back the ACK frame to the STA5 (stepS123). At this point in time, the transmission-suspended state of theSTA3 and the STA4 is cancelled, and changed to the transmission-enabledstate.

Subsequently, after having sent back the ACK frame to the STA5 (stepS123), the AP2 transmits the RTR frame to the STA4 (step S124). On theother hand, the STA3 is turned to the transmission-suspended state basedon the NAV included in the RTR frame at step S124, for example, untilthe reception finish time of the CTS frame from the AP2, (step S125).

The STA4 having received the RTR frame in the processing at step S124transmits the RTS frame to the AP2 in response thereto (step S126). Onthe other hand, the STA5 is turned to the transmission-suspended state,for example, until the reception processing of the STA4 finishes, basedon the NAV included in the RTS frame at step S126 (step S127).

The AP2 having received the RTS frame in the processing at step S126transmits the CTS frame to the STA4 (step S128). On the other hand, theSTA3 extends the transmission-suspend-period, according to need, basedon the NAV in the CTS frame addressed to the STA4 (step S129).

The STA4 having received the CTS frame in processing at step S128 readsthe DATA frame temporarily stored in the processing at step S110 andtransmits the DATA frame to the AP2 (step S130). Lastly, the AP2 havingreceived the DATA frame sends back the ACK frame to the STA4 (stepS131). At this point in time, the transmission-suspended state of theSTA3 and the STA5 is cancelled, and changed to the transmission-enabledstate.

In this embodiment, a case that a hidden terminal appears due to a radioterminal adjacent to the base station, which operates in the samefrequency, has been explained. However, for example, the same procedurecan be applied to a case that a radio terminal cannot performtransmission processing, because the base station operating in the samefrequency operates within the interference range. Furthermore, even whenthe base station has received a frame from a radio terminal connected bycarrier sense, but immediately after the reception, transmissionprocessing is disabled, the same procedure can be applied bytransmitting the RTR frame when the base station is turned to thetransmission-enabled state.

Furthermore, even in a sequence in which the RTS/CTS procedure as themeasures against hidden terminals and the interference problem is notperformed, the same procedure can be applied by transmitting the RTRframe. In this case, the processing at steps from S106 to S109, and fromS111 to S114 is replaced by the retransmission processing of the DATAframe, and after the transmission of the RTR frame, only the procedureat step S122 (DATA frame) and step S123 (ACK frame), and at step S130(DATA frame) and step S131 (ACK frame) is performed. Furthermore, whilethe access method based on the CSMA/CA has been explained in thisembodiment, the same procedure can be also applied to the access methodbased on the polling control, by reserving polling time within the RTRframe and reserving delivery of the reception frame. In this case, afield for delivery reservation is added to the RTR frame. In theconfiguration of the communication system, it is assumed herein that thebase station can be a particular radio terminal, and a radio terminalcan be the base station. When the priority level of the DATA frametemporarily stored in the buffer drops with the lapse of time, the DATAframe stored in the buffer can be cancelled, or rescheduled for thetransmission at the next time onward. Such a case is handled by adding afield indicating the priority level to the RTR frame of the DATA frame.

In the sixth embodiment, when a particular base station is in thetransmission-suspended state due to the influence of a hidden terminalor interference, in spite of having an access from a radio terminal, thebase station sequentially transmits the RTR frame for requesting theradio terminal having data of the highest priority level to retransmitthe DATA frame, which could not be received, when the base station isturned to the transmission-enabled state, so that the respective radioterminals sequentially retransmit the past DATA frame. Accordingly, anuplink packet from a plurality of radio terminals can be efficientlyreceived.

INDUSTRIAL APPLICABILITY

The communication method of the present invention is useful for adigital radio communication system adopting the CSMA, which is one ofthe random access technology, and particularly, suitable as a method foravoiding a collision of wireless packets.

1. A communication method for a carrier-sense-multiple-access (CSMA)network including a radio terminal and a base station to which the radioterminal belongs, the communication method supportingrequest-to-send/clear-to-send (RTS/CTS) to prevent a collision ofpackets due to a hidden terminal, the communication method comprising:RTS-transmitting including the base station transmitting arequest-to-send (RTS) frame to the radio terminal during atransmission-suspend-period in which the radio terminal suspendstransmission to prevent the collision of packets; wherein when the basestation does not receive a clear-to-send (CTS) frame from the radioterminal due to a suspension of the transmission, and when there isanother data frame to be transmitted to another radio terminal, the basestation performs a communication with the another radio terminal inpriority to a communication with the radio terminal and the radioterminal extends the transmission-suspend-period based on a usage periodfor which the another radio terminal uses a channel; RTR-transmittingincluding the radio terminal transmitting a request-to-receive (RTR)frame to the base station after the transmission-suspend-period haselapsed; and data-transmitting including the base station transmitting adata frame to the radio terminal in response to the RTR frame.
 2. Thecommunication method according to claim 1, further comprising storingincluding the base station temporarily storing the data frame to betransmitted to the radio terminal.
 3. The communication method accordingto claim 2, wherein the storing includes the base station storing thedata frame after the base station transmits the RTS frame to the radioterminal for a predetermined number of times.
 4. The communicationmethod according to claim 1, wherein when a plurality of base stationsor a plurality of other radio terminals transmit a plurality of RTSframes, respectively, to the radio terminal during thetransmission-suspend-period, the RTR-transmitting includes the radioterminal transmitting the RTR frame to the base stations or to the otherradio terminals sequentially in descending order of priority.
 5. Thecommunication method according to claim 1, further comprises: the basestation transmitting a request-to-send (RTS) frame to the radio terminalin response to the RTR frame; wherein the data-transmitting includingthe base station does not transmit a data frame to the radio terminal inresponse to the RTR frame; the radio terminal transmitting aclear-to-send (CTS) frame to the base station in response to the RTSframe; the base station transmitting the data frame to the radioterminal in response to the CTS frame; and the radio terminaltransmitting an acknowledgement (ACK) frame after having received thedata frame.
 6. The communication method according to claim 1, furtherincludes: the radio terminal transmitting an acknowledgement (ACK) frameafter having received the data frame.
 7. The communication methodaccording to claim 5, wherein another radio terminal, which receives theRTR frame from the radio terminal or communicates with the base stationthat has received the RTR frame from the radio terminal, suspendstransmission to prevent the collision of packets until thedata-transmitting is completed.
 8. The communication method according toclaim 6, wherein another radio terminal, which receives the RTR framefrom the radio terminal or communicates with the base station that hasreceived the RTR frame from the radio terminal, suspends transmission toprevent the collision of packets until the data-transmitting iscompleted.
 9. A radio terminal employing carrier-sense-multiple-access(CSMA) with request-to-send/clear-to-send (RTS/CTS) to prevent acollision of packets due to a hidden terminal, the radio terminalcomprising: an RTS-receiving unit that receives a request-to-send (RTS)frame from a base station, to which the radio terminal belongs, during atransmission-suspend-period in which the radio terminal suspendstransmission to prevent the collision of the packets; and anRTR-transmitting unit that transmits a request-to-receive (RTR) frame tothe base station after the transmission-suspend-period has elapsedwherein when the RTS-receiving unit receives a plurality of RTS framesfrom a plurality of base stations or from a plurality of other radioterminals during the transmission-suspend-period, the RTR-transmittingunit transmits the RTR frame to the base stations or to the other radioterminals sequentially in descending order of priority.
 10. The radioterminal according to claim 9, further comprising an extending unit thatextends, when the base station performs a communication with anotherradio terminal in priority to a communication with the radio terminalduring the transmission-suspend-period, the transmission-suspend-period,based on a usage period for which the another radio terminal uses achannel.
 11. The radio terminal according to claim 9, furthercomprising: a CTS-transmitting unit that transmits a clear-to-send (CTS)frame to the base station in response to another RTS frame that istransmitted from the base station in response to the RTR frame; and anACK-transmitting unit that transmits an acknowledgement (ACK) frame tothe base station after having received a data frame that is transmittedfrom the base station in response to the CTS frame.
 12. The radioterminal according to claim 9, further comprising an ACK-transmittingunit that transmits an acknowledgement (ACK) frame to the base stationafter having received a data frame that is transmitted from the basestation in response to the RTR frame.
 13. A base station employingcarrier-sense-multiple-access (CSMA) with request-to-send/clear-to-send(RTS/CTS), the base station comprising: an RTS-transmitting unit thattransmits a request-to-send (RTS) frame to a radio terminal during atransmission-suspend-period in which the radio terminal suspendstransmission to prevent a collision of packets; an RTR-receiving unitthat receives a request-to-receive (RTR) frame from the radio terminalafter the transmission-suspend-period has elapsed; an RTS-retransmittingunit that retransmits the RTS frame to the radio terminal in response tothe RTR frame; and a data-transmitting unit that transmits a data frameto the radio terminal in response to a clear-to-send (CTS) frame that istransmitted from the radio terminal in response to the RTS frameretransmitted.
 14. The base station according to claim 13, furthercomprising a buffer that temporarily stores the data frame to betransmitted to the radio terminal.
 15. The base station according toclaim 14, wherein the buffer stores the data frame after theRTS-transmitting unit transmits the RTS frame to the radio terminal fora predetermined number of times.
 16. The base station according to claim13, wherein when the CTS frame is not transmitted from the radioterminal in response to the RTS frame transmitted due to a suspension ofthe transmission, and when there is another data frame to be transmittedto another radio terminal, the base station performs a communicationwith the another radio terminal in priority to a communication with theradio terminal.
 17. A base station employingcarrier-sense-multiple-access (CSMA) with request-to-send/clear-to-send(RTS/CTS), the base station comprising: an RTS-transmitting unit thattransmits a request-to-send (RTS) frame to a radio terminal during atransmission-suspend-period in which the radio terminal suspendstransmission to prevent a collision of packets; wherein when aclear-to-send (CTS) frame is not transmitted from the radio terminal inresponse to the RTS frame transmitted due to a suspension of thetransmission, and when there is another data frame to be transmitted toanother radio terminal, the base station performs a communication withthe another radio terminal in priority to a communication with the radioterminal, the radio terminal extends the transmission-suspend-periodbased on a usage period for which the another base station or theanother radio terminal uses a channel; an RTR-receiving unit thatreceives a request-to-receive (RTR) frame from the radio terminal afterthe transmission-suspend-period has elapsed; and a data-transmittingunit that transmits a data frame to the radio terminal in response tothe RTR frame.
 18. The base station according to claim 17, furthercomprising a buffer that temporarily stores the data frame to betransmitted to the radio terminal.
 19. The base station according toclaim 18, wherein the buffer stores the data frame after theRTS-transmitting unit transmits the RTS frame to the radio terminal fora predetermined number of times.
 20. A communication method for acarrier-sense-multiple-access (CSMA) network including a radio terminaland a base station to which the radio terminal belongs, thecommunication method supporting request-to-send/clear-to-send (RTS/CTS)to prevent a collision of packets due to a hidden terminal, thecommunication method comprising: RTS-transmitting including the radioterminal transmitting a request-to-send (RTS) frame to the base stationduring a transmission-suspend-period in which the base station suspendstransmission to prevent the collision of packets or due to aninterference; wherein and when the radio terminal does not receive aclear-to-send (CTS) frame from the base station due to a suspension ofthe transmission, and when there is another data frame to be transmittedto another base station or to another radio terminal, the radio terminalperforms a communication with the another base station or with theanother radio terminal in priority to a communication with the basestation, the base station extends the transmission-suspend-period basedon a usage period for which the another base station or the anotherradio terminal uses a channel; RTR-transmitting including the basestation transmitting a request-to-receive (RTR) frame to the radioterminal after the transmission-suspend-period has elapsed; anddata-transmitting including the radio terminal transmitting a data frameto the base station in response to the RTR frame.
 21. The communicationmethod according to claim 20, further comprising storing including theradio terminal temporarily storing the data frame to be transmitted tothe base station.
 22. The communication method according to claim 21,wherein the storing includes the radio terminal storing the data frameafter the radio terminal transmits the RTS frame to the base station fora predetermined number of times.
 23. The communication method accordingto claim 20, wherein when a plurality of other base stations or aplurality of radio terminals transmit a plurality of RTS frames,respectively, to the base station during thetransmission-suspend-period, the RTR-transmitting includes the basestation transmitting the RTR frame to the other base stations or to theradio terminals sequentially in descending order of priority.
 24. A basestation employing carrier-sense-multiple-access (CSMA) withrequest-to-send/clear-to-send (RTS/CTS) to prevent a collision ofpackets due to a hidden terminal, the base station comprising: anRTS-receiving unit that receives a request-to-send (RTS) frame from aradio terminal belonging to the base station during atransmission-suspend-period in which the base station suspendstransmission to prevent the collision of the packets or due to aninterference; and an RTR-transmitting unit that transmits arequest-to-receive (RTR) frame to the radio terminal after thetransmission-suspend-period has elapsed wherein when the RTS-receivingunit receives a plurality of RTS frames from a plurality of other basestations or from a plurality of radio terminals during thetransmission-suspend-period, the RTR-transmitting unit transmits the RTRframe to the other base stations or to the radio terminals sequentiallyin descending order of priority.
 25. The base station according to claim24, further comprising: a CTS-transmitting unit that transmits aclear-to-send (CTS) frame to the radio terminal in response to anotherRTS frame that is transmitted from the radio terminal in response to theRTR frame; and an ACK-transmitting unit that transmits anacknowledgement (ACK) frame to the radio terminal after having receiveda data frame that is transmitted from the radio terminal in response tothe CTS frame.
 26. A communication method for acarrier-sense-multiple-access (CSMA) network including a radio terminal,a first base station, and a second base station, the communicationmethod supporting request-to-send/clear-to-send (RTS/CTS) to prevent acollision of packets due to a hidden terminal, the communication methodcomprising: transmitting including the second base station transmittinga frame for communication between base stations to the first basestation during a transmission-suspend-period in which the first basestation suspends transmission to prevent the collision of packets or dueto an interference; wherein when the second base station does notreceive a response to the frame for communication between base stationsfrom the first base station due to a suspension of the transmission, andwhen there is another data frame to be transmitted to a third basestation or to the radio terminal, the second base station performs acommunication with the third base station or with the radio terminal inpriority to a communication with the first base station, the first basestation extends the transmission-suspend-period based on a usage periodfor which the third base station or the radio terminal uses a channel;transmitting including the first base station transmitting arequest-to-receive (RTR) frame to the second base station after thetransmission-suspend-period has elapsed; and transmitting including thesecond base station transmitting a data frame to the first base stationin response to the RTR frame.
 27. The communication method according toclaim 26, further comprising storing including the second base stationtemporarily storing the data frame to be transmitted to the first basestation.
 28. The communication method according to claim 27, wherein thestoring includes the second base station storing the data frame afterthe second base station transmits the frame for communication betweenbase stations to the first base station for a predetermined number oftimes.
 29. A base station employing carrier-sense-multiple-access (CSMA)with request-to-send/clear-to-send (RTS/CTS) to prevent a collision ofpackets due to a hidden terminal, the base station comprising: areceiving unit that receives a frame for communication between basestations from another base station during a transmission-suspend-periodin which the base station suspends transmission to prevent the collisionof the packets or due to an interference; and a transmitting unit thattransmits a request-to-receive (RTR) frame to the another base stationafter the transmission-suspend-period has elapsed wherein when thereceiving unit receives a plurality of frames for communication betweenbase stations from a plurality of other base stations during thetransmission-suspend-period, the transmitting unit transmits the RTRframe to the other base stations sequentially in descending order ofpriority.
 30. The base station according to claim 29, furthercomprising: the transmitting unit that transmits a clear-to-send (CTS)frame to the another base station in response to another frame forcommunication that is transmitted from the another base station inresponse to the RTR frame; and the transmitting unit that transmits anacknowledgement (ACK) frame to the another base station after havingreceived a data frame that is transmitted from the another base stationin response to the CTS frame.